As I read the engine combo it sounded like a good, conservative combo...
Then I saw those tiny and long headers on a 542 CID I would love to see the dyno difference between those headers and your stepped 2 to 2-1/8" dyno headers.
So I am thinking 690 hp at a low peak rpm ~6200, but it might not fall off too bad. I hope it makes more. I wish I knew what Mark Gate's engine made, since the heads are from that engine.
